Answer:
The original concentration is "35 mg/ml".
Explanation:
According to the question,
The solution is diluted,
= 1:50
The initial volume,
V1 = 1 ml
Final concentration,
= 0.07 mg
then,
The final volume,
V2 = 500 ml
As we know,
⇒ 
or,
⇒ 
On substituting the values, we get
⇒ 
⇒ 
⇒ 
It has the same composition throughout, unlike a heterogeneous which has visibly separate substances
I believe Mendeleev formed his periodic table by atomic weight (nowadays we do it by atomic number) so he likely used those values.